The process of dynamic adsorption, for removing an impurity or adsorbate from a liquid or gas stream, is achieved by directing the fluid stream through a column of a suitable adsorbent. The adsorbent may be molecular sieves, activated aluminum, silica gels, fuller's earth, activated carbon or other materials known, or found, to be useful for the intended purpose.
Commonly the adsorbent is contained in an elongated tank and the fluid stream passes through the material from one end of the tank to the other end. The majority of the impurity removal occurs in a limited zone, which is called a mass transfer zone (MTZ). The MTZ will be of a definite length for each intended application. The beginning of the zone is defined as the point where the fluid has a 95% concentration of the inlet adsorbate concentration. The end of the zone is the point where the fluid stream has a 5% concentratoin of the inlet adsorbate concentration. The adsorbate may be any undesired contaminate, including organic materials, acids, caustic materials, alcohols, aromatics, water and other undesired substances.
The column of adsorbent and dynamic adsorption tanks is generally of a length which is a multiple (preferably at least three) of the length of the MTZ for the intended application. This enables the MTZ to move along the column as more and more of the adsorbent at the inlet becomes "spent" or reaches equilibrium with the incoming fluid. The zone of "spent" adsorbent, therefore, increases with time while the zone below the MTZ, where the adsorbent is only slightly contaminated, and is essentially in equilibrium with the exit fluid, shrinks with time. By providing a column length that is several times as long as the MTZ, a longer operating time is obtained for the filter before replacement or regeneration is required.
For a given mass flow rate, temperature, pressure and adsorbate concentration, it is known that tall, thin columns have higher dynamic adsorption capacities than short, thick ones. This means that more efficient dynamic adsorption is obtained by using high velocity, long adsorbent paths.
The tanks used for filters that employ the MTZ principle have separate inlet and outlet connections at opposite ends of the filter tank which require threaded elements which may be relatively expensive and inconvenient to connect. Other tanks are permanently mounted to the system and the adsorbent must be periodically drained and replaced. In addition, additional support structure must be provided to support these tanks.
Oil filters made for automotive applications are commonly constructed with a centrally located output port that is threaded for spin-connection to an output pipe and an outer ring of input port aptitude which receive the contaminated oil. This type of filter has achieved great acceptance because of the ease with which it may be replaced, its relatively large filtering capacity for its size and because it is self-supporting when it is screwed into place. The flow pattern of the conventional oil filter, however, is from the inlet port through a small portion of the filter media, which differs for different parts of the fluid stream, and out a multitude of small holes that are provided along substantially the entire length of the inner core which leads to the outlet port. While this type of fluid flow is suitable for particulate oil filtration, it does not provide for the optimum purification of fluids that can be obtained with a given amount of adsorbent material, in accordance with the present invention. For example, filtration accomplished by the present invention is especially well suited for the purification of fluids employed in semiconductor manufacture, where the fluid purification process may be selected for particular types of impurities and the concentration of these impurities in the exit fluid must be exceedingly small, especially where vacuum pump systems are involved. It is also useful for removing impurities from compressed gas or air streams.